Certain writing instruments in prior art comprise an outer casing which defines a longitudinal throughhole, a refill extending through the throughhole, a nose cone threadably engaged on the outer casing, and a rear casing is removably inserted into an end of the throughhole.
A cam body is fixed on the outer casing inside the throughhole and has a plurality of cam surfaces and cam grooves. A rotating cam is arranged in the casing throughhole to receive a refill, and is engageable by the edge of the cam grooves. A cam bar is arranged and adapted to push the rotating cam forwardly.
When a user pushes the rear casing forwardly, the longitudinal movement via the cam bar cause the rotating cam to push forwardly and rotate at predetermined angles by the cooperation of the rotating cam and the cam surfaces.
Because the rear casing is removable with respect to the outer casing, however, such an instrument creates the risk that a child may swallow it and suffocate.
In order to eliminate the risk, some prior art instruments are provided with rear casings which have a hole to ensure an air flow introduced therethrough when swallowed. However it is high in cost.